Polycyclic carbon structures

In summary, based on the available experimental and theoretical data, neither the existence nor the nonexistence of non-carbon fullerene polymorphs can be concluded. However, since sp bonding is energetically more favorable for Cn than for SIn, in the latter there are no caps and no plane structures which play a crucial role in fullerene formation. It seems that the mechanism of formation of such complexes for carbon and silicon is different. Taking into account the fact that the fullerene structure is not the global minimum for either carbon or silicon and in combination with the observable tendency of silicon clusters to have a spherical form, one can assume that under certain conditions processes similar to Stone-Wales rearrangement from polycyclic carbon structures to fullerene is also possible for silicon. Currently there is no common opinion on this issue. However, in addition to the notes about direct syntheses of non-carbon fullerenes, it is possible to specify some new synthetic approaches to the formation of silicon clusters. 

One of the most interesting types of polycyclic carbon compounds prepared in recent years is the group of tricyclic substances known as propellanes. A typical example is tricyclo[3.2.2.0 -5]nonane, which sometimes is called [3.2.2]propellane, 12. The physical properties of several of these are included in Table 12-6. A quick look at formula 12 probably does not suggest any great structural difference from the bicyclic compounds we have discussed previously. However, if one tries to construct a ball-and-stick model of 12, one soon concludes that the propellanes are truly extraordinary substances in that all four carbon bonds at the bridgehead carbons extend, not to the comers of a tetrahedron, or even a distorted tetrahedron as for a cyclopropane ring, but... 

These data, in conjunction with previous studies (12) showed that stable char contains mainly condensed aromatic structure with intermittent paraffinic groups. This structure is formed by successive dehydration, rearrangement, loss of carboxyl, carbonyl, and paraffinic groups, formation of free radicals, and condensation of the carbon skeleton to polycyclic aromatic structures. 

Flame retardants may not only catalyze dehydration of the cellulose to more char and fewer volatiles but also enhance the condensation of the char to form cross-linked and thermally stable polycyclic aromatic structures (60). Cellulose was treated with various additives and then charred at 400 °C. The chars were then oxidized with permanganate see Chapter 13) and the results are in Table IV. The char yield was slightly higher for the sodium chloride-treated sample (17.5%) and substantially more for the sample containing diammonium phosphate (28.9%), as compared to the yield from the untreated sample (15.3%). Furthermore, the increased char formation was accompanied by increased aromaticity, as measured by the amount of the aromatic carbon obtained from the char and the amount obtained from the original cellulose molecules (60). 

A corollary of this statement is the following If these polyaromatic or polycyclic saturated structures are present in the carbon skeleton of coal, they should be identified in the short-contact-time liquefaction products. The possibility of some isomerization reactions in the carbon skeleton cannot be excluded totally, but the most important fact is that no dramatic aromatization of hydroaromatic rings or saturation of aromatic rings takes place under these conditions. Many of the chemical functions also are stable under these conditions, especially the O, S, and N heterocyclic aromatic structures. Water formation by phenol dehydroxylation is minimal. In coal liquefaction under our conditions, even at long reaction times (up to 90 min) in the absence of an added catalyst, the -OH bonded to a monoaromatic ring is stable. Under the same conditions, dehydroxylation of polyaromatic phenols does occur (10). 

NIPU networks are created by the reaction between polycyclic carbonate oligomers and aliphatic or cycloaliphatic polyamines with primary amino groups [4], This forms a cross-linked polymer with p-hydroxy urethane groups of a different structure—polyhydroxyurethane polymer. Since NIPU is obtained without using highly toxic isocyanates, the process of synthesis is relatively safe for both humans and the environment in comparison to the production of conventional polyurethanes. Moreover, NIPU is not sensitive to moisture in the surrounding environment. 

A plausible nucleation mechanism could be the gradual formation of carbon structures For instance by formation of linear carbon chains which via the addition of other linear chains and the addition of small carbon radicals can grow to bear graphite-like polycyclic networks. 

There has been a recent attempt to combine MP2/ 6-31G //HF/6-31G calculations with simple carbon substructure parameters as an accurate approach to correlating and predicting the enthalpies of formation of cata- and peri-condensed unsubstituted polycyclic aromatic hydrocarbons. The electronic energies provide the nonadditive electronic effects, and the carbon-structure parameters model the additive effects. The quality of the multilinear regression was... 

Another strategic device applies specifically to polycyclic compounds. In the interests of simplification we want to remove some of the rings and give an intermediate with a famihar ring structure. We can do this by the common atom approach. In TM 329, mark all the carbon atoms which belong to more than one ring - the common atoms . 

In general the most stable resonance structure for a polycyclic aromatic hydro carbon is the one with the greatest number of rings that correspond to Kekule formula tions of benzene Naphthalene provides a fairly typical example... 

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

Alicyclic Hydrocarbons. These refer to cyclic analogues of aliphatic hydrocarbons and are named accordingly, using the piefix cyclo-." Their properties are similar to their open-chain aliphatic counterparts. Alicyclic hydrocarbons are subdivided into monocyclic (cycloalkanes, cycloalkenes, cycloalkynes, cycloalkadienes, etc.) and polycyclic aliphatic compounds. Monocyclic aliphatic structures having more than 30 carbon atoms in the ring are known, but those containing 5 or 6 carbon atoms are more commonly found in nature [47, p. 28]. 

Strychnine, the most celebrated member of the Strychnos alkaloids, possesses a complex polycyclic structure which is assembled from only twenty-four skeletal atoms. In addition to its obvious architectural complexity, strychnine s structure contains a contiguous array of six unsymmetrically substituted tetrahedral (asymmetric) carbon atoms of which five are included within one saturated six-membered ring. The intimidating structure of the strychnine molecule elicited the following remark by Sir Robert Robinson in 1952 For its molecular size it is the most complex substance known. 5... 

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